Optimum ignition conditions for gas turbine engines are subject to variation between starts due to a variety of factors, e.g. ambient temperature, gas turbine temperatures, fuel calorific value, fuel content, pressures, repeatability of fuel and air delivery systems, etc. In a typical start system for a gas turbine engine one uses an auxiliary drive unit for driving the gas turbine and a control unit providing a start sequence in which gas turbine speed and fuel delivery are coordinated to provide a fuel/air mixture at an ignition device allowing a successful ignition.
In a typical start sequence, the speed of the gas turbine, which is during start driven by an auxiliary motor, and/or the fuel flow to the combustion system are progressively increased over a set period of time, the so called light-up window. The length of the light-up window is a function of the range of engine speeds at which starting is most likely to occur, typically between 5% and 20% of the rated engine speed and the accumulation rate of fuel in the combustor. During the light-up window a number of ignition opportunities appears, the actual number of which depends on the number of sparks that can be delivered per second by the igniter of the gas turbine engine and the length (duration) of the light-up window. Therefore one likes to have the light-up window as long as possible. However, the length of the light-up window is delimited by a number of factors. If, e.g. the turbine is accelerated too quickly the fuel injection system will not have enough time to provide a sufficient amount of fuel before the window of engine speeds at which starting is most likely to occur is exceeded. On the other hand, if the turbine is accelerated too slowly, it may happen that an amount of fuel inside the combustor is reached which could be dangerous to the engine while the turbine speed has still not reached the maximum speed within the light-up window. However, for example the acceleration rate of the turbine depends on the ambient conditions. On a cold day, a battery driven starter motor may not be capable of accelerating the engine quickly due to possible low power supply. On the other hand, on a very hot day, the same motor with the same battery may be capable of accelerating the engine very quickly. To cope with the mentioned limitations a compromise is typically required between maximizing the light-up window to cover for wide variations in the actual optimum window and minimizing the variation rate to increase the number of ignition opportunities (sparks) during the actual optimum window, without establishing a potentially dangerous fuel amount inside the combustion system during the light-up window. Such a start sequence for gas turbine engines is, e.g. described in US 2010/0293960 A1 or U.S. Pat. No. 7,878,004 B1.
In cases when the gas turbine has more than one burner the situation is even more complicated. A burner has a significant difference in a so called air flow pressure loss coefficient before (cold condition) or after (hot condition) it is ignited due to aerodynamic air blockage effects of the combusting hot burner. Generally speaking, a cold burner has a lower air pressure loss coefficient than a hot burner. Burners are optimized to have high ignition reliability within the designed ignition or light-up window of fuel to air ratio. However, due to the hot and cold burner loss coefficient difference the already ignited burner (s) has/have a higher loss coefficient and therefore less air will flow through this/these hot burner (s). Consequently, more air will flow to the unignited burner (s). This maldistribution of air between hot and cold burners in the engine causes the unignited burner (s) to have more air pass through and a higher air to fuel ratio. This effect force (s) the unignited burner (s) to operate outside the optimised ignition or light-up window and therefore in a failed and problematic engine start.